1. Field of the Invention
The present invention relates to a dielectric resonator device in which a plurality of resonators are formed in a cavity, and also relates to a communication filter and a communication unit for a mobile communication base station using the above type of dielectric resonator device.
2. Description of the Related Art
A known dielectric resonator device, which is used in a filter, formed by providing a plurality of dielectric resonators in a cavity is disclosed in Japanese Unexamined Patent Application Publication No. 7-321506.
In this publication, a plurality of dielectric cores, each having two dielectric rectangular parallelepipeds intersecting with each other, are provided in cavities so as to form a plurality of TM double mode dielectric resonators. This publication also discloses a structure in which a partitioning plate having a plurality of slits in a predetermined direction is disposed between adjacent TM double mode dielectric resonators so that magnetic coupling of the same type of mode of the double modes is conducted between adjacent dielectric resonators. By the provision of this partitioning plate, magnetic fields oriented along the length of the slits are allowed to pass through the partitioning plate, while magnetic fields oriented along the width of the slits are shielded, thereby making it possible to couple the same predetermined type of resonant mode.
The direction in which magnetic fields are allowed to pass through the partitioning plate can be determined by the number, the width, and the aspect ratio of the slits, etc. If, for example, the width of the slits is decreased, the effect of shielding magnetic fields orienting along the width of the slits can be enhanced. However, the magnetic fields cannot be completely shielded, and the resonators disposed across the partitioning plate are slightly coupled to each other due to such magnetic fields. If such a coupling is undesirable, a predetermined filter characteristic cannot be obtained. It is also difficult to reduce the width of the slits formed in the partitioning plate to further than a predetermined value in terms of the manufacturing process.
Additionally, the ratio of the permeability of magnetic fields along the length of the slits to that along the width of the slits can be changed to a certain degree by the aspect ratio. However, this permeability ratio is fixed by the shape of the slits formed in the partitioning plate. Accordingly, the optimal coupling coefficient between adjacent resonators cannot be separately adjusted for the two modes in which magnetic fields are directed along the length of the slits and magnetic fields are directed along the width of the slits.